1. Field of the Invention
The present invention relates to a semiconductor device and a liquid crystal display device. In particular, the invention relates to a semiconductor device and a liquid crystal display device each controlling liquid crystal molecules by generating an electric field almost parallel to a substrate.
2. Description of the Related Art
One of technical development strategies of a liquid crystal display device is widening a viewing angle. As a technique for realizing a wide viewing angle, a mode has been used in which gray scales are controlled by generating an electric field almost parallel to a substrate to move liquid crystal molecules within a surface parallel to the substrate. IPS (In-Plane Switching) and FFS (Fringe-Field Switching) are given as such a mode. In an FFS mode, a second electrode having an opening pattern (e.g., a pixel electrode of which a voltage is controlled per pixel) is provided below liquid crystals, and a first electrode (e.g., a common electrode of which a voltage common to all pixels is supplied) is provided below the opening pattern. An electric field is applied between the pixel electrode and the common electrode, so that the liquid crystals are controlled. Since an electric field is applied to the liquid crystals in a direction parallel to a substrate, liquid crystal molecules can be controlled by using the electric field. That is, since liquid crystal molecules which are aligned in parallel to the substrate (so-called homogeneous alignment) can be controlled in a direction parallel to the substrate, a viewing angle is widened.
The first electrode (the common electrode) is formed to be in direct contact with a glass substrate, and a gate electrode in an inversely staggered transistor is also formed to be in direct contact with the glass substrate. An insulating film functioning as a gate insulating film in the inversely staggered transistor is formed to be in direct contact therewith. In addition, the second electrode (the pixel electrode) is formed thereover (see Reference 1: Japanese Published Patent Application No. 2000-89255).
Alternatively, the first electrode (the common electrode) is formed to be in direct contact with the insulating film functioning as the gate insulating film in the inversely staggered transistor. Note that a semiconductor film, a source electrode, and a drain electrode are also formed to be in direct contact with the insulating film functioning as the gate insulating film in the inversely staggered transistor. In addition, an insulating layer is formed to be in direct contact therewith. Further, the second electrode (the pixel electrode) is formed thereover (see Reference 1: Japanese Published Patent Application No. 2000-89255).